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Astron. Astrophys. 325, 745-754 (1997)

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3. Brief description of the observations

Till now there exist 5 HST spectral intensity measurements of the interplanetary hydrogen H-Ly [FORMULA] resonance glow at 5 different targets taken at 5 different times (see Table 1). The data are high resolution registrations from 1212.3 Å  to 1218.7 Å  measured by the GHRS on board the HST during extended exposure time periods.


[TABLE]

Table 1. HST measurement data


The surveys were made in the 'FP-SPLIT' mode of the GHRS instrument. For every position and view direction, where H-Ly [FORMULA] data are available, between 4 and 10 data sets exist, with an integration time of 544 sec each (with few exceptions of shorter integration times). With appropriate IDL-software procedures these data sets belonging to one target will all be merged resulting in noise-reduced spectra for every view direction. (Details of the GHRS instrument and data handling ('FP-SPLIT' mode) in Baum (1994), or Soderblom (1994)).

3.1. The Doppler shift

The HST is an earth-bound satellite, thus, all data are strongly influenced by the Doppler shift caused by both the HST orbital motion with respect to the earth and of the ecliptical motion of the earth around the sun. The velocity components of these motions relative to the line of sight (LOS) have to be taken into account when comparing the data with theoretical radiative transport calculations (Sect. 4).

3.1.1. The HST orbital motion

In all measured data the geocoronal H-Ly [FORMULA] glow, as the strongest spectral feature, is seen (Fig. 6). Assuming that the earth's atmosphere is fixed to the earth, the Doppler shift along LOS caused by the HST orbital motion around the earth is identical for the geocorona and the interplanetary H-Ly [FORMULA] glow. By determining the shift of the geocoronal spectra measured by the GHRS with respect to geocoronal rest frame and by a corresponding shift of the data, the influence of the HST orbital motion on the spectral location of the interplanetary H-Ly [FORMULA] glow data are eliminated. (Barycentric motion moon-earth only plays a minor role and will be neglected).

3.1.2. The ecliptical motion of the earth

For each day for which HST data were available the earth's velocity vector was obtained by taking the derivatives of the J2000 ephemeris coordinates; the Earth-Moon motion and the sun-barycenter motion have not been taken into account. The resulting velocity vector has an accuracy of 0.025 km/s within the epoch range of 1900 to 2100 AD. Transforming the earth's velocity vector and the GHRS instrument view direction in cartesian coordinates for every day of measurement, the earth velocity component into the LOS direction is given by the scalar product of these two vectors (see Table 1).

The remaining shift of the H-Ly [FORMULA] background glow is caused by the ("Doppler-shift" projection of the) velocity pattern of the interplanetary hydrogen to the GHRS line of sight and is taken into account by the radiation transport model (Sect. 4).

3.2. The GHRS instrument function

The instrument point spread function (PSF) of the GHRS describes how an actual monochromatic point source is spectrally broadened by the electrical and optical instrument environment and by the optical slit-spectrometer mounting. This instrument function has been determined by several authors (e.g.: Clarke et al. 1995; Gilliland et al. 1992) (Fig. 1).

Because the real observational data are very noisy, it is not advised here to deconvolve the data from this PSF. Instead it is more convenient to convolve the model results with this PSF and compare these results with the data (Sect. 6). The calculated spectra (Sect. 4) are similar to a Voigt profile with a temperature of the order of [FORMULA] K. After the convolution of the calculated spectra with the GHRS PSF function (with [FORMULA] 0.1 Å, see Fig. 1) the resulting spectra are then like a Voigt profile with a temperature of [FORMULA] K (see Fig. 5).

[FIGURE] Fig. 1. Point Spread Function of the HST GHRS instrument (from Clarke et al. 1995).

[FIGURE] Fig. 2. Position and view direction of the GHRS HST instrument. (note: view 4 [FORMULA] view 1, view 5 [FORMULA] view 3, see Table 1). The hydrogen inflow direction is adopted with [FORMULA] and [FORMULA] (Lallement et al. 1993)

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© European Southern Observatory (ESO) 1997

Online publication: April 28, 1998

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